1. Field of the Invention
The present invention relates to a constant voltage generating circuit for selectively generating a constant voltage, and more particularly to a constant voltage generating circuit for quickly generating a constant voltage upon request thereof.
2. Description of the Related Art
Constant voltage generating circuits have been widely used for generating constant voltages which are usually utilized as a reference voltages in various fields. For example, in a dynamic memory device, a constant voltage generating circuit is employed for generating a reference voltage used for precharging bit lines. Namely, in a dynamic memory device employing a one-transistor type memory cell, each pair of bit lines are precharged to a reference voltage having an intermediate level of the logic "1" and "0" levels prior to a read operation. A read-out signal from a selected memory cell is applied to one of a pair of bit lines to compare its potential with the reference voltage precharged to the other bit line of the same pair for determining a logic level of the read-out signal. Since the constant voltage generated by the generating circuit is not continuously utilized but is used for only a predetermined period. Therefore, such a constant voltage generating circuit that produces a constant voltage in a controlled manner when the output of the constant voltage is necessary.
A conventional constant voltage circuit of this type is constructed by an output series circuit having a first field effect transistor connected between a first voltage terminal and an output terminal and a second field effect transistor connected between the output terminal and a second voltage terminal and a control circuit for rendering the first and second transistors conductive thereby to produce a constant voltage at the output terminal in a first mode and non-conductive to set the output terminal at a high impedance state in a second mode. The control circuit includes resistors, field effect transistors and switch elements connected in series between the first an second voltage terminals. The switch elements are made conductive in the first mode so that a DC current flows through the control circuit to bias the gates of the first and second transistors, while the switch elements are made non-conductive in the second mode to disenable the bias circuit so that the gates of the first and second transistors are set at inactive levels. When the constant voltage generating circuit switched from the second mode to the first mode, the gates of the first and second transistors are charged to their predetermined bias voltages through the resistors of the control circuit. However, in order to reduce a power consumption due to a current flowing through the bias circuit, the values of the resistors of the control circuit are made large. It requires a relatively large time to precharge the gates of the first and second transistors after the transistion from the second mode to the first mode. Accordingly, the conventional constant voltage generating circuit has a long response time to generate the constant voltage.